WBR1105

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Pulmonology
Prompt	[[Prompt::A 35-year-old female immigrant presents to the physician's for shortness of breath that occurs when she travels to the farm. Upon further questioning, the patient informs the physician that she belongs to the Kayan tribe of Burma that is known for a tradition in which the women gradually extend the length of their necks with coils. Prior to performing a methacholine challenge test, the physician obtains baseline measurements for the following: Tidal volume = 600mL PaCO 2 = 40 mmHg P expired CO 2 = 28 mmHg Respiratory Rate = 16/min Which of the following represents the physiologic dead space in this patient?]]
Answer A	AnswerA::150 mL
Answer A Explanation	AnswerAExp::150mL is approximately the normal physiologic dead space, but the dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.
Answer B	AnswerB::175 mL
Answer B Explanation	AnswerBExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.
Answer C	AnswerC::180 mL
Answer C Explanation	[[AnswerCExp::Although the approximately normal physiologic dead space is 150 mL, the patient has an unusual anatomy that may increase the volume of her physiologic dead space. The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.]]
Answer D	AnswerD::190 mL
Answer D Explanation	AnswerDExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.
Answer E	AnswerE::225 mL
Answer E Explanation	AnswerEExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette is being evaluated for asthma after reporting feeling pulmonary symptoms on exposure to a foreign stimulus (the farm). In this case, the patient has a unique anatomy due to the tribe’s tradition of using series of coils to extend their necks. Dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it remains in the conducting airways, or reaches alveoli that are not perfused or poorly perfused. We are asked to calculate the physiologic dead space in this patient. Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution it is possible to measure anatomical and alveolar dead space, employing the concept of mass balance, as expressed by Bohr equation (below). VD = dead space VT = tidal volume PaCO2 = partial pressure of carbon dioxide in arteries PECO2 = partial pressure of carbon dioxide in exhaled air In normal individuals, physiologic dead space is approximately 150mL. However, when using the values given in the stem to the Bohr equation, the physiologic dead space in this patient is increased to 180 mL. Educational Objective: The physiologic dead space can be calculated with the Bohr equation, which quantifies the using the dilution of alveolar CO2 as it is exhaled. VD = dead space VT = tidal volume PaCO2 = partial pressure of carbon dioxide in arteries PECO2 = partial pressure of carbon dioxide in exhaled air References: First Aid 2015 page 602 First Aid 2014 page 596]]
Approved	Approved::Yes
Keyword	WBRKeyword::Lung, WBRKeyword::Lung volume, WBRKeyword::Dead space, WBRKeyword::Pulmonology, WBRKeyword::Physiology, WBRKeyword::Respiratory physiology
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